1cfj

METHYLPHOSPHONYLATED ACETYLCHOLINESTERASE (AGED) OBTAINED BY REACTION WITH O-ISOPROPYLMETHYLPHOSPHONOFLUORIDATE (GB, SARIN)
(see also AChE inhibitors and substrates and 1som)

 Acetylcholinesterase (AChE) hydrolysizes the neurotransmitter acetylcholine (ACh), producing choline and an acetate group. ACh directly binds catalytic Ser200 (via its nucleophilic Oγ atom). Soman, O-(1,2,2-trimethylpropyl) methylphosphonofluoridate (fluorine atom is colored violet and phosphorus atom is colored darkmagenta ), is one of the most toxic organophosphate compounds (OPs). Soman inhibits AChE by covalent binding to catalytic Ser200, mimicking ACh. This process (phosphonylation) implicates nucleophilic attack of the Ser200 nucleophilic Oγ atom on the phosphorus atom of soman, with concomitant departure of its fluoride atom. After that AChE catalyzes the dealkylation ("aging") of the soman or other OP. This causes irreversible inhibition of AChE, "aged" soman/AChE conjugate can not be reactivated. However, before “aging”, at the step of phosphonylation, AChE can be reactivated by nucleophiles, such as pralidoxime (2-PAM), resulting in cleavage of the phosphonyl adduct from Ser200 Oγ. At the active site of the nonaged soman/TcAChE conjugate (2wfz) the catalytic His440 forms hydrogen bonds with Ser200 Oγ and Glu327 Oε1 via its Nε2 and Nδ1 nitrogens, respectively. The O2 atom of soman is within hydrogen bonding distance of His440 Nε2. Soman O1 mimicks carbonyl oxygen of ACh. A water molecule 1001 interacting with soman O2 is represented as a <font color='red'>red ball. The active site residues of the nonaged soman/TcAChE are colored <font color='yellow'>yellow. The O2 atom of the <scene name='2wfz/Ali/4'>dealkylated (aged) soman (2wg0) forms a salt bridge with His440 Nε2. The active site residues of the aged soman/TcAChE are colored <font color='pink'>pink. <scene name='2wfz/Ali/5'>Alignment of the structures of the nonaged (2wfz) and aged (2wg0) conjugates reveals a small, but important, change within the active site - the imidazole ring of His440 is tilted back to a native-like conformation after dealkylation. The water molecule 1001, which interacts with soman O2 in the nonaged crystal structure, is not within hydrogen bonding distance of O2 in the aged crystal structure. 2-PAM binds poorly to the nonaged phosphonylated enzyme (its electron density was not found) and binds in an <scene name='2wfz/Ali/7'>unfavorable and nonfunctional conformation after soman aging to TcAChE (2wg1).

<applet load='1cfj.pdb' size='500' frame='true' align='left' scene='1cfj/Cv/1' /> Sarin, O-isopropylmethylphosponofluoridate, is an other toxic OP compound. It is also inhibits AChE by covalent binding to the catalytic Ser200. The active sites of aged <scene name='1cfj/Cv/2'>sarin-TcAChE (1cfj) and aged soman-TcAChE (1som and 2wg0) are almost identical and provided structural models for the negatively charged, tetrahedral intermediate that occurs during deacylation with the ACh. There are four hydrogen bond donors <font color='red'>(red dotted lines) to the anionic phosphonyl oxygen atoms: the backbone amide nitrogen atoms of Ala201, Gly118, and Gly119, as well as His440 Nε2. The sarin <font color='cyan'>methyl carbon (colored cyan) is within non-bonded contact distances (black dotted lines) of Phe288 and Phe290 in the acyl binding pocket.

About this Structure
1CFJ is a 1 chain structure of sequence from Torpedo californica. The June 2004 RCSB PDB Molecule of the Month feature on Acetylcholinesterase by David S. Goodsell is 10.2210/rcsb_pdb/mom_2004_6. Full crystallographic information is available from OCA.

Additional Resources
For additional information, see: Alzheimer's Disease